The NFL has played the national anthem before games for decades,
but it wasn't until 2009 that it started playing the song with
the players on the field.

In 2016, Colin Kaepernick, a quarterback who at the time was
playing for the San Francisco 49ers, protested police treatment
of minorities by sitting during the anthem. The move divided and
politicized the NFL. The controversy had mostly faded from
conversation this season as Kaepernick has been without a team,
but it flared back up after Trump spoke out against players who
did not stand for the anthem.